Measuring the thickness of printed products during the assembly process is necessary to establish which products have not been put together completely, i.e., which are missing individual pages, parts, and/or supplements. Conventionally, the thickness of printed products is determined by direct measurement of the products themselves. The present invention measures thicknesses utilizing laser beams which detect the relative location of two sensors' distances measured with great accuracy. Prior art thickness detection devices are arranged at various locations along production lines for assembling multi-sheet or multi-part printing products to repeatedly detect and reject defective products. An example of such a device is described in EP-A-0 479 717 and the corresponding U.S. Pat. No. 5,154,279.
It is therefore an object of the present invention to provide a process and a device making it possible to accurately measure the thickness of printed products even when the printed products are moving at high speeds along the production line.
It is a further object of this invention to determine the thickness of printed products being transported during production past the sensor which quickly determines the thickness of the product with great accuracy. The thickness of the printed products may be used to establish whether the printed products have been assembled incompletely; that is, which pages, parts, or supplements are missing, while on the other hand indicating which products contain too many pages, parts, or supplements.